In recent years, in rapidly developing mobile communication systems (for example, Personal Handyphone System, hereinafter referred to as “PHS”), in order to make efficient use of frequencies, a variety of methods for controlling transmission channel allocation have been proposed, of which some have been put into practical use.
In particular, recently, a PDMA system has been proposed to improve efficiency in the use of radio wave frequencies as mobile telephones have come into widespread use. In the PDMA system, one time slot of the same frequency is spatially divided for data transmission of a plurality of users.
In the PDMA system, one time slot is spatially divided into a plurality of channels using a mutual interference canceller such as an adaptive array, allowing a plurality of users who will cause little interference with each other to establish path multiple access to the time slot.
More specifically, in the PDMA system, a cell station (hereinafter, referred to as a “CS”) separates and extracts a multiple streams of signal waves from personal stations (hereinafter, referred to as a “PS”) of a plurality of users who have established path multiple access to channels of the same frequency and the same time slot, using well-known adaptive array processing.
FIG. 9 schematically shows path multiple access state between a CS and PSs in the PDMA system. Referring to FIG. 9, assume that, for example, three PSs, that is, a PS 10 of a user A, a PS 20 of a user B and a PS 30 of a user C have initially established path multiple access to a CS 100.
Here, if PS 10 is disconnected from CS 100, as shown by an arrow (X) for a reason such as movement of user A out of area or deterioration of a transmission path due to interference and the like, PS 10 will try to establish a connection with another CS 200 (what is called “handover”), as shown by a dashed line and an arrow (Y).
Meanwhile, the previous connection target CS 100, that no longer can receive a radio wave from PS 10, transmits at a prescribed level, a synchronous burst for reestablishing connection to PS 10, that is, a synchronous burst for reconnection, for a period of several tens of seconds, as schematically shown by an arrow (Z).
If PS 10 was not able to establish connection to CS 200 for some reason, PS 10 will try to recover connection with CS 100. The above described synchronous burst for reconnection serves as a mark in such a case. Therefore, if PS 10 can recognize the synchronous burst for reconnection (Z), PS 10 will re-establish synchronization with CS 100, and moreover, re-establish a communication channel with CS 100 through a prescribed sequence. On the other hand, if PS 10 was not able to recognize the synchronous burst for reconnection (Z), PS 10 cannot re-establish synchronization with CS 100, and fails in connection with any CS. Consequently, the communication channel is disconnected.
Here, generally, when a connection between a CS and a PS has been established, a radio wave is transmitted from the CS to the PS with transmission directivity through well-known adaptive array processing, and the radio wave will not interfere with other PSs that have established multiple access to the CS. As described in the example of FIG. 9, however, for example, while PS 10 has been disconnected from CS 100 for handover to CS 200 for a period of several seconds or several tens of seconds, CS 100 is not receiving a signal wave from PS 10, and transmission directivity to PS 10 has been lost. In other words, the synchronous burst for reconnection transmitted at a prescribed level without transmission directivity to PS 10 for several tens of seconds is simply a disturbance wave (an interfering wave) for respective PSs 20 and 30 of other users B and C that have established path multiple access to CS 100.
Therefore, transmission from the CS, of the synchronous burst for reconnection without directivity for a long period of time will have an adverse effect on radio waves of other PSs connected to the CS.
Thus, an object of the present invention is to provide a radio base station system capable of alleviating an effect of a synchronous burst for reconnection on other PSs that have established multiple access to the CS, and capable of easily recovering connection of the PS to the CS, that has failed in handover, as well as to provide a method and a program for controlling transmission of such a synchronous burst.